The invention relates to a network feedback unit and an electrical drive system.
The invention is based on the object of providing a network feedback unit and an electrical drive system by means of which energy, for example braking energy, can be fed into and fed back from a network in a reliable manner and with a high degree of efficiency.
The invention achieves this object by way of a network feedback unit and an electrical drive system in accordance with embodiments of the invention.
The network feedback unit is designed to feed electrical energy from an intermediate circuit into a three-phase network.
The network feedback unit has a buck converter unit. The buck converter unit has a first buck converter and a second buck converter. It goes without saying that the buck converter unit can also have more than two buck converters. In the following text, the invention is described by way of example for two buck converters. The input side and the output side of the first buck converter and the second buck converter are connected in parallel and the input side of each of which is electrically coupled to the voltage intermediate circuit.
The network feedback unit furthermore has an inverter, the input side of which is electrically coupled to an output of the buck converter unit and the output side of which is electrically coupled to the three-phase network. The inverter is typically a current-fed or voltage-fed commutator.
The network feedback unit furthermore has at least one filter capacitor, which is arranged at the output of the buck converter unit. Alternatively or in addition, one or more filter capacitors can be arranged at the output of the inverter.
The network feedback unit furthermore has a controller unit, for example in the form of a microprocessor or FPGA and associated hardware and software, which is designed to drive or operate the first buck converter and the second buck converter depending on a filter capacitor current, that is to say a current flowing through the filter capacitor, in such a way that the first buck converter and the second buck converter contribute on average over time in equal parts to an output current of the buck converter unit. In the ideal case, the first buck converter and the second buck converter are driven in such a way that the first buck converter and the second buck converter each deliver on average over time approximately half of the output current of the buck converter unit. If the network feedback unit has n buck converters, a respective buck converter delivers 1/nth of the output current of the buck converter unit, wherein n represents a natural number and n>1 holds true.
The buck converters can each be operated based on pulse-width modulation, wherein a duty cycle within a respective (PWM) period of the pulse-width modulation is conventionally set for this purpose, wherein the duty cycle influences an output current of the respective buck converter, for example. The buck converters can be operated within a respective period in a staggered manner. If the buck converter unit has, for example, two buck converters, the two buck converters can be clocked in a manner staggered by 180 degrees. For this case, the controller unit can be designed to drive or operate the first buck converter and the second buck converter depending on the filter capacitor current in such a way that the first buck converter and the second buck converter contribute on average over time over a respective period of the pulse-width modulation in equal parts to an output current of the buck converter unit.
The filter capacitor can be arranged at the output of the buck converter unit, for example looped in or connected between output connection poles of the buck converter unit. The network feedback unit can have a current sensor, in particular arranged in the filter current path, which current sensor is designed to measure the filter capacitor current.
The network feedback unit can have a number of filter capacitors at the output of the inverter, for example exactly three filter capacitors. The network feedback unit can have an, in particular corresponding or identical, number of current sensors, wherein the current sensors are each designed to measure an associated filter capacitor current. A first current sensor can measure, for example, a first filter capacitor current, a second current sensor can measure, for example, a second filter capacitor current and a third current sensor can measure, for example, a third filter capacitor current.
The controller unit can have an intermediate circuit voltage controller, which is designed to regulate an intermediate circuit voltage to a setpoint value. The intermediate circuit voltage controller can output an output current setpoint value for the buck converter unit as a manipulated variable. The controller unit can furthermore have a buck converter unit current controller, which is designed, depending on a difference between the output current setpoint value and the output current (actual value) of the buck converter unit and also depending on a balancing signal, to generate a first current setting signal and a second current setting signal. The first current setting signal in this case determines an output current of the first buck converter and the second current setting signal accordingly determines an output current of the second buck converter. The controller unit can have a balancing signal generation unit, which is designed to generate the balancing signal depending on the filter capacitor current in such a way that the balancing signal is dependent on the parts in which the first buck converter and the second buck converter contribute to the output current of the buck converter unit.
The first buck converter and the second buck converter can each have at least one semiconductor switching means, in particular exactly two semiconductor switching means. The controller unit can have a first PWM generation unit, wherein the first PWM generation unit is designed to generate a first pulse-width-modulated drive signal for the at least one semiconductor switching means of the first buck converter depending on the first current setting signal. The controller unit can furthermore have a second PWM generation unit, wherein the second PWM generation unit is designed to generate a second pulse-width-modulated drive signal for the at least one semiconductor switching means of the second buck converter depending on the second current setting signal. The controller unit can furthermore have a current sensor, which is designed to measure the filter capacitor current. The balancing signal generation unit can have a comparator, which is operatively connected to the current sensor, said comparator being designed to output a comparator signal having a first logic state, for example one, when the filter capacitor current is greater than an upper threshold value and to output the comparator signal having a second logic state, for example zero, when the filter capacitor current is lower than a lower threshold value. The upper and the lower threshold value can be identical or can differ in order to implement hysteresis. The balancing signal generation unit can furthermore have a logic unit having at least one logic gate, at least one integrating element and/or at least one sampling element, wherein the logic unit is supplied with the comparator signal and the pulse-width-modulated drive signals and is designed to generate the balancing signal depending on the comparator signal and the pulse-width-modulated drive signals.
The first buck converter and the second buck converter can each have a first commutation cell and a second commutation cell, wherein the first commutation cells have a capacitor, a diode and a semiconductor switching means, wherein the second commutation cells accordingly have a capacitor, a diode and a semiconductor switching means, wherein the capacitor of the first commutation cells and the capacitor of the second commutation cells are looped in series between input connection poles of the buck converter unit, wherein the controller unit has a voltage balancing unit, which is designed to drive the semiconductor switching means of the first commutation cells and the semiconductor switching means of the second commutation cells in such a way that identical voltages are set at the capacitor of the first commutation cells and the capacitor of the second commutation cells.
The network feedback unit can have an output current determination unit, which is designed to identify or synthesize the output current of the buck converter unit depending on the filter capacitor current or the filter capacitor currents and depending on phase output currents, which are fed or impressed into the three-phase network by means of the inverter.
The output current determination unit can have a selection unit, which is supplied with the phase output currents and a selection signal and which is designed to output a single one of the phase output currents depending on the selection signal. The output current determination unit can furthermore have a summer, which is supplied with the filter capacitor current and the phase output current output by the selection unit and which is designed to sum the filter capacitor current and the phase output current output by the selection unit, wherein the resulting sum represents the output current.
Alternatively, the output current determination unit can have a first selection unit, which is supplied with the phase output currents and a selection signal and which is designed to output a single one of the phase output currents depending on the selection signal, and a second selection unit, which is supplied with the filter capacitor currents and the selection signal and which is designed to output a single one of the filter capacitor currents depending on the selection signal. The output current determination unit can furthermore have a summer, which is supplied with the phase output current output by the first selection unit and with the filter capacitor current output by the second selection unit and which is designed to sum the selected phase output current and the selected filter capacitor current, wherein the resulting sum represents the output current.
The electrical drive system has: at least one electrical drive, at least one frequency converter for driving the electrical drive, wherein the frequency converter has an intermediate circuit and/or is electrically coupled to an intermediate circuit, and a network feedback unit, which is described above and coupled to the intermediate circuit and a three-phase network.